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Alumaflex ducting vs flex ducting in attic

I was not able to figure out which forum was the right one to ask this in. I'm a DIY homeowner (ex contractor) and the general forum asks homeowners to post in the AOP forums, and the AOP forums say no DIY.

I have a home in the SF bay area with an uninsulated attic. We have mild weather of course. The attic hosts a gas furnace and most of it's ductwork is what I call alumaflex - the core is thin aluminum, wrapped in a bit of fiberglass and plastic sheathing/skin. Most of the sheathing has disintegrated and is hanging down. Some of the fiberglass wrapping is hanging down too. This stuff is about 20 years old.

I would like to replace much of this ducting. The question is should I use alumaflex, or go with flex ducting (that is, wire coil, with insulated wapper)? I was going to use alumaflex because I'd guess it sends the air down with less resistance. But flex ducting is a lot easier to work with and cheaper. Could I just install a larger diameter duct - ie where now it has 8" alumaflex, install 10" flex? The idea being that the larger size would compensate for the increased resistance of the flex duct.

In some areas the ducting will lay on the rafters, in others it will need to be suspended.

the pic I found showed grey outer liner, the type that disintergrates
in sunlight. if that is what you have, then switching to R-8
duct with reflective outer liner would be a better choice.

if flex is pulled tight & sized correctly it moves air just as well as
hard pipe ducting. keeping the duct straight with sweeping turns
will allow air to flow properly.
I use a 3" duct strap, and flex needs to have no more than a 1"
sag per 4' of install.

prior to taking duct into attic..it takes two people to pull the inner liner,
otherwise the inner liner will restrict air flow.
use only mastics, and mastic tapes to seal ducts.

best of luck.

The cure of the part should not be attempted without the cure of the whole. ~Plato

Thanks for that input...thus I'll probably opt for flex, make sure it's as straight as possible, and pull the inner lining in advance (tho I don't quite envision the issue, I'll pay attention to the flex duct when I buy it and I'm sure it will make sense).

One interesting bit of info that many installers need to understand is that flex ducts need to have the inner liner pulled tight. If it's not tight, the friction rate jumps significantly, and that means the air flow takes a hit. The diagram below shows that if a flex duct has even 15% longitudinal compression, the friction rate doubles. With 30% longitudinal compression, the friction rate quadruples.

The article mentions that it is ideal to use solid ducting at corners. If you take the idea of stretching the inner lining tight seriously, solid ducting at bends starts to seem almost imperative. If you have a 90 degree bend in a continuous run of flex, and the inner lining is tight, the corner will probably be too tight; ie it'll be a very sharp bend. This makes things more complicated obviously.

I wonder if using the alumaflex type material would be enough at the corners, or if they mean solid as in steel sheet metal? I'm not sure the aluminum flex ducting can handle being an end point for well pulled flex ducting.

if flex is pulled tight & sized correctly it moves air just as well as
hard pipe ducting.

I don't believe this statement is entirely accurate. Even tightly stretched flex duct is going to be more restrictive than sheet metal duct.

"The bitterness of poor quality remains long after the sweetness of low price is forgotten". --Benjamin Franklin
"Don't argue with an idiot, they will drag you down to their level and beat you with experience". --Mark Twainhttp://www.campbellmechanical.com

I don't believe this statement is entirely accurate. Even tightly stretched flex duct is going to be more restrictive than sheet metal duct.

You are correct sir. A "properly" installed flex duct has the same friction loss as a round duct 2" smaller in diameter and I have rarely seen installers take the time to do it right. Also you should use mastic on the inside of the liner before sliding it over the metal collar so you will have a good seal. Panduit straps are worthless for the attaching the liner to the collar.

Should use a SS draw band. Use the panduit straps for attaching the insulation to the collar. Panduit will stretch with heat and soon become loose and your inner will either blow off of the collar or it will leak bad. SMACNA recommends using metal elbows by the way. I do have an old (can't get them anymore) flex ductulator which shows that a fully stretched flex has the same friction loss and velocity as a round duct 2" smaller and same for a 6" to 4" as a 14" to a 12" as a matter of fact. So do what you want but if you use flex up the sizes by 2" keep it tight, use metal elbows and hope it is installed right which probably won't happen. Thank you, thank you very much

"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
"I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."Barry Goldwater

I don't believe this statement is entirely accurate. Even tightly stretched flex duct is going to be more restrictive than sheet metal duct.

air1,
you are right.
thanks for catching my error.

alumaflex and flex would be comperable, not flex &
hard pipe.
although flex done correctly does performs well,
it is the restrictions, excess duct & bad installs that seem
to label it a cheap bad install. flex done correctly is a good thing.

OP, gentle sweeping bends rather than tight bends.

not mentioned in the article is use of 90 degree ells at supply
boxs. this is something I always try to use as it keeps air flow
at supply box from becomming restrictive. attaching the ell to
the supply box, and attaching the flex over the ell is a good
idea. imo.

these and all connections should be mastic sealed.
use of paint on mastic, if time allows or mastic tape
if time is short.

best of luck.

The cure of the part should not be attempted without the cure of the whole. ~Plato

Want to pass on a little info on pressure losses in flex duct. You will not like this study but in short it says that if you don't stretch flex to the very max you are drastically increasing the pressure loses through the flex duct. If you just have a 4% compression in your flex then the pressure loss for a 8" duct at 200 CFM is about 10% higher than a 7" steel duct. Now if you had a 15% compression on the duct which isn't much then you are looking at the a pressure loss close to a 5" steel duct. Check it out. Thank you very much

"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
"I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."Barry Goldwater

it is for this reason that flex installers need to take the time to streach out the inner liner
before even going into attic with flex.
otj..I've had people look at me like I'm crazy for doing this.
they understood nothing about pressure losses.
if left alone each run would be 25' with no excess cut off.
& ducts lying on insulation of attic floor.

thanks for the link...another one for the download file!

The cure of the part should not be attempted without the cure of the whole. ~Plato

I've seen installs like ERLA describes; looks like a giant snake up in the attic. Distance between main metal duct trunk and the supply register was 10 feet, and they used the whole box of flex to get there.

An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............http://www.mohomeenergyaudits.com